The present invention relates to a suspension control system for use in a vehicle.
One example of conventional suspension control systems is disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 7-232530.
The system shown in the above-mentioned publication has a shock absorber of the variable damping characteristic type interposed between sprung and unsprung members of a vehicle; an actuator for changing damping force generated by the shock absorber; a vertical acceleration sensor for detecting the vertical acceleration acting on the sprung mass of the vehicle; a velocity detecting means for detecting the vertical velocity of the sprung mass of the vehicle; and a controller for controlling the actuator by delivering to the actuator a control signal for obtaining damping force corresponding to the velocity signal from the velocity detecting means.
The controller previously stores damping force-current (control signal) characteristics of the shock absorber with respect to one piston speed (e.g. P1 in FIG. 6) as shown in FIG. 25, by way of example. The controller supplies the actuator with an electric current having a magnitude corresponding to the control signal, thereby causing the shock absorber to generate damping force having a magnitude corresponding to the control signal. Further, the controller judges the road surface condition according to the degree of acceleration detected by the vertical acceleration sensor and changes the control gain for the control signal to adjust the control signal according to the road surface condition.
Incidentally, damping force generated by the shock absorber varies with the travel speed of the piston provided in the shock absorber, as shown in FIG. 6. In the prior art, on the assumption that the piston is traveling at one piston speed P1 (e.g. 0.3 m/S), the controller outputs an electric current (control signal) that generates damping force needed at the piston speed of 0.3 m/S.
In this regard, the actual piston speed is changing at all times. However, when damping force control is needed during running on an ordinary road or a slightly rough road (i.e. when the vehicle body is moving upward or downward at a velocity more than a predetermined value), the piston speed is about 0.3 m/S on the average. Therefore, satisfactory control effect is obtained.
However, when the vehicle runs on a rough road or an extremely rough road, the piston speed increases. Consequently, even when the control signal is the same, generated damping force may increase depending upon the surface condition of the road on which the vehicle is running. Therefore, there are cases where the expected control effect cannot be obtained. With the above-described prior art, if damping force control is effected on the basis of the damping force-current (control signal) characteristics shown in FIG. 25 without taking into consideration the piston speed, there may be an excess or deficiency of damping force according to the piston speed such that when the piston speed is high, the damping force becomes excess (over-control), whereas when the piston speed is low, a deficiency of damping force occurs (under-control).
It should be noted that the above-described prior art allows the control gain to be changed according to the piston speed (road surface condition), thereby making it possible to generate damping force as desired to a certain extent according to the piston speed.
The change of the control gain in the above-described prior art gives rise to no problem if the damping characteristics change linearly. However, the actual damping characteristics are non-linear as shown in FIG. 6. Therefore, the damping force control taking into consideration the piston speed suffers low accuracy and is likely to result in over-control or under-control.
The present invention was made in view of the above-described circumstances. Accordingly, an object of the present invention is to provide a suspension control system capable of generating appropriate damping force in accordance with variations in the piston speed.
The present invention has a shock absorber of the variable damping characteristic type interposed between sprung and unsprung members of a vehicle; an actuator for changing the damping characteristics of the shock absorber; a velocity detecting device for detecting the velocity of the sprung member; a piston speed estimating device for estimating the level of speed of a piston performing sliding movement inside the shock absorber; and a controller having a plurality of damping force maps showing correspondence relation between damping force and an actuator command signal, respectively in correspondence to various levels of the piston speed. The controller obtains necessary damping force from the velocity of the sprung member. Further, the controller selects a damping force map corresponding to the piston speed level estimated by the piston speed estimating device and outputs the actuator command signal on the basis of the selected damping force map.
According to one aspect of the present invention, the piston speed estimating device is a road surface condition detecting device for detecting the road surface condition, and the controller selects a damping force map according to the road surface condition detected by the road surface condition detecting device.
In this case, a sprung vibration detecting device for detecting the acceleration frequency of the sprung member may be provided, so that the road surface condition detecting device detects the road surface condition according to the value of the acceleration frequency.
According to another aspect of the present invention, the piston speed estimating device is a vehicle behavior detecting device for detecting the behavior of the vehicle, and the controller selects a damping force map according to the behavior of the vehicle detected by the vehicle behavior detecting device. The behavior of the vehicle may be dive of the vehicle, squat of the vehicle, or rolling of the vehicle.
According to another aspect of the present invention, the piston speed estimating device is a vehicle position information acquiring device for acquiring information about the position of the vehicle from an external communication device, and the controller selects a damping force map according to the vehicle position information acquired by the vehicle position information acquiring device.
In any of the above-described various modes, a vehicle speed detecting device for detecting the vehicle speed may be provided, so that the controller adds vehicle speed information detected by the vehicle speed detecting device as information for selecting a damping force map.